Having worked in a coffee shop where we did allow outside food due to the location, I can totally understand places banning it. Aside from the fact that anything people bring in is one thing they’re not buying from you, people would bring full-on picnics, leave mounds of trash, borrow cutlery and then buy one drink between the 5 of them. I honestly had to spend half an hour cleaning up after one such group, they’d spilt stuff everywhere, even ground food into the seats. The one drink they bought didn’t even pay my wages for cleaning up after them.
Other places I’ve worked had an official ‘no outside food’ policy, but if one member of a group brought their own stuff due to allergies or something, or finished off the meal with fruit or something, that was OK. Policy depended on customer attitude (and manager mood).
There’s also a possibility someone objecting to you bringing your own water over their free table water has that policy in case drinks bought in aren’t actually water. Vodka-filled water bottles would not be a first, and it’s not always the people you’d suspect either.